1. Field of the Invention
The present invention relates to a liquid crystal device using a nematic liquid crystal having twisted alignment state.
2. Description of the Related Art
Liquid crystal devices are widely used as display devices for television sets, wordprocessors, computer terminals, personal computers, etc. Liquid crystal devices are classified into a simple matrix type and an active matrix type. The former type has pixels formed respectively at points where electrodes arranged in row on one substrate and electrodes arranged in column on another substrate cross one another via a liquid crystal. The latter type has pixels formed respectively at positions where a common electrode formed on one substrate faces segment electrodes arranged on another substrate, and an active element (thin film transistor) is provided for each pixel.
For such matrix type liquid crystal display devices, there are demands for a large display and improvement of resolution. Therefore, the number of pixels arranged per display device is significantly increased. This demands recent liquid crystal devices to be driven in a high-duty multiplex manner.
A twisted nematic liquid crystal display device (hereinafter referred to as TN-LCD) which has a relatively high response speed and a relatively high contrast is used as a display device that, like a television set, displays motion pictures. This TN-LCD comprises a pair of substrates; a nematic liquid crystal sealed between the substrates with the directions of its molecular axes sequentially twisted 90.degree. between the substrates; and polarizing plates respectively disposed outside the substrate pair. The 90.degree. twisted arrangement is realized as follows. The inner surfaces of the pair of substrates facing each other with a given gap therebetween are subjected to aligning treatment. The aligning treatment comprises the steps of forming aligning films on the mentioned substrate surfaces in order to provide the nematic liquid crystal with alignment restrictive force, and rubbing the surfaces of the aligning films in order to align the axes of the liquid crystal molecules in a given direction. Accordingly, those of the liquid crystal molecules near the substrates of the aligning films have the molecular axes aligned to be nearly parallel to the rubbing direction. Hereinafter, the direction of the liquid crystal molecules aligned by the alignment restrictive force of the aligning treatment is defined as a aligning treatment direction. FIG. 1 illustrates the relation between the aligning treatment direction of the substrate pair and the polarizing axes (including an absorbing axis and a transmitting axis) of the polarizing plates disposed outside the substrates. In FIG. 1, as indicated by a broken arrow 3, the direction of aligning treatment of the lower substrate 1 is inclined 45.degree. to the edge of the substrate 1, from the upper left of the substrate 1 in the diagram toward the lower right. As indicated by a solid arrow 4, the direction of aligning treatment of the upper substrate 2 is inclined 45.degree. to the edge of the substrate 2, from the lower left (in the diagram) toward the upper right. With light being transmitted from the lower substrate toward the upper substrate 2, the aligning treatment direction of the upper substrate 2 differs 90.degree. clockwise in the light-traveling direction (as viewed from the back of the diagram sheet) from that of the lower substrate 1.
As a result, the liquid crystal sealed between the lower substrate 1 and the upper substrate 2 is aligned in such a manner that the molecular axes are parallel to the broken arrow 3 at the proximity of the lower substrate 1, and are aligned in the direction of the solid arrow 4 at the proximity of the upper substrate 2. The middle liquid crystal molecules are aligned in such a manner that their axes are sequentially twisted counterclockwise, as viewed in the light-traveling direction. Therefore, the liquid crystal is aligned, twisted 90.degree. counterclockwise between the lower and upper substrates 1 and 2. The direction of the polarizing axes (including an absorbing axis and a transmitting axis) of the polarizing plates disposed outside the substrates 1 and 2 is set as illustrated in FIG. 1. Specifically, the direction of the polarizing axis of the lower polarizing plate disposed outside the lower substrate 1 is set parallel to the aligning treatment direction of the lower substrate 1 as indicated by a broken arrow 5. The direction of the polarizing axis of the upper polarizing plate disposed outside the upper substrate 2 is set in a direction that crosses at 90.degree. the direction of the polarizing axis of the lower polariz-ing plate, as indicated by a solid arrow 6. To enhance the contrast, .DELTA.n.multidot.d, the product of the optical anisotropy .DELTA.n of the liquid crystal and the layer thickness d (gap) thereof, is set nearly to 1.
The above-described TN-LCDs have a relatively flat spectral distribution, can attain a light transmitting state (open) in which the liquid crystal is hardly colored and a light shielding state (close), and has a relatively high contrast. Due to this feature, the TN-LCDs are widely used as various display devices. When the duty of multiplex drive becomes high (i.e., when the number of time divisions increases), however, the operation margin decreases, thus reducing the contrast. It is therefore difficult to provide a highduty multiplex drive for the TN-LCDs.
This reduction in operation margin in a TN-LCD occurs due to poor sharpness of the threshold characteristic of the TN-LCD (ratio of a change in luminance to the applied voltage; thereinafter referred to as .gamma. characteristic). The .gamma. characteristic can be improved by increasing the ratio of a change in aligning state to the applied voltage. To realize this, it has been proposed to increase the twist angle and decrease the elastic constant. As liquid crystal devices having the twist angle set to 180.degree. to 360.degree., there are a super twisted nematic liquid crystal device (hereinafter referred to as STN-LCD) having a relatively small twist angle and a super birefringence effect liquid crystal device (hereinafter referred to as SBE-LCD) having a relatively large twist angle. The SBE-LCD is disclosed in U.S. Pat. No. 4,697,884 and U.S. Pat. No. 4,634,229. FIG. 2 illustrates the direction of aligning treatment of the substrate pair and the directions of the polarizing axes of the polarizing plates in the SBE-LCD. Referring to this diagram, the direction of the aligning treatment of a lower substrate 7 is inclined about 45.degree. in the lower right direction to the lower edge of the substrate 7, as indicated by a broken arrow 8. The direction of the aligning treatment of an upper substrate 9 is deviated 270.degree. clockwise from the arrow 8 (the direction of a solid arrow 10), as viewed from the back of the diagram sheet. Accordingly, the molecular axes of the molecules of the liquid crystal sealed between the two substrates is twisted 270.degree. counterclockwise from the aligning treatment direction (broken arrow 8) of the lower substrate 7 toward the aligning treatment direction (solid arrow 10) of the upper substrate 9, as viewed from the back of the diagram sheet. The polarizing axis of the polarizing plate disposed outside the lower substrate 7 is set in a direction of a broken arrow 11, deviated 45.degree. from the aligning treatment direction (broken arrow) of the substrate 7. The polarizing axis of the polarizing plate disposed outside the upper substrate 9 is set in a direction of a solid arrow 12, deviated 45.degree. from the aligning treatment direction (solid arrow 10) of the substrate 9. Between both substrates is sealed a liquid crystal for which .DELTA.n.multidot.d, the product of the optical anisotropy .DELTA.n and the layer thickness d, is 0.78 .mu.m or 0.84 .mu.m.
As described above, the SBE-LCD and STN-LCD have the .gamma. characteristic improved by increasing the twist angle and has the visual contrast enhanced by utilizing the double refraction birefringence effect. Due to their large twist angle, however, these two LCDs have a low response time. Because of the use of the double refraction birefringence effect, there would occur peaks in the spectral distribution of transmitting light, as shown in FIG. 6, due to the wavelength dependency of the refractive index, and the display face would be colored purplish blue in close state and yellowish green in open state, as indicated by the CIE chromaticity chart. Although this liquid crystal device is suitable for character display, therefore, it is not suitable for display of a motion picture such as a TV image. Nor is the liquid crystal device suitable for a color display.
To overcome the problems of the SBE-LCD and STN-LCD, it is proposed to prevent coloring of the display face by reducing the product .DELTA.n.multidot.d of the optical anisotropy .DELTA.n of the liquid crystal sealed between the two substrates and the thickness d of the liquid crystal layer (hereinafter referred to as retardation .DELTA.n.multidot.d). This liquid crystal device was reported as an optical mode interference effect liquid crystal device (hereinafter referred to as OMI-LCD) in Appl. Phys., lett. 50(2), 2 Feb. 1987, and SID DIGEST 1987, p. 372-375, by M. Schadt and F. Leenhouts, et al.
FIG. 3 illustrates the aligning treatment direction of the surfaces of a pair of substrates and the direction of polarizing axes of polarizing plates in this OMI-LCD. Referring to this diagram, the aligning treatment direction of a lower substrate 13 is the direction of a broken arrow 14 parallel to the lower edge of the substrate, and the aligning treatment direction of a upper substrate 15 is the direction of a solid arrow 16 parallel to the aligning treatment direction of the lower substrate 13. As a result, the liquid crystal sealed between both substrates has the molecular axes twisted 180.degree. clockwise from the aligning treatment direction (broken arrow 14) of the lower substrate 13, as viewed from the back of the diagram sheet. The direction of the polarizing axis of the polarizing plate disposed outside the lower substrate 13 is set in the direction of a broken arrow 17 parallel to the aligning treatment direction (broken arrow 14) of the lower substrate 13. The direction of the polarizing axis of the polarizing plate disposed outside the upper substrate 15 is set in the direction of a solid arrow 18 normal to the direction of the polarizing axis of the lower polarizing plate (broken arrow 17). Between both substrates is sealed a liquid crystal having a retardation .DELTA.n.multidot.d of about 0.55 .mu.m.
This OMI-LCD has .DELTA.n.multidot.d set significantly small in order to suppress coloring of the display face, thus ensuring achromatic color display. This OMI-LCD however has a low transmittivity in light-transmitting state to realize the achromatic color display and has a poor .gamma. characteristic.
Various modifications of the STN-LCD and OMI-LCD have been proposed. Given that the aligning treatment direction of a lower substrate 19 in these LCDs is indicated by a broken arrow 20 and the direction of the aligning treatment direction of an upper substrate 21 is indicated by a solid arrow 22, as illustrated in FIG. 4, the twist angle .alpha..degree. of the liquid crystal sealed between these substrates is set between 160.degree. and 360.degree.; an angle .alpha. formed between the aligning treatment direction (broken arrow 20) of the lower substrate 19 and the direction of the polarizing axis of the lower polarizing plate disposed outside the substrate 19 (indicated by a broken arrow 23) is set between 0.degree. and 110.degree.; and with the direction of polarizing axis of the polarizing plate disposed outside the upper substrate 21 being indicated by a solid arrow 24, the crossing angle between the upper and lower polarizing plates is set between 0.degree. and 90.degree.. And a liquid crystal having a retardation .DELTA.n.multidot.d of 0.4 to 0.6 or 0.7 to 1.2 is sealed between the upper and lower substrates 21 and 19.
These LCDs are substantially the same as the aforementioned STN-LCD and/or OMI-LCD and still have the mentioned shortcomings: coloring of the display face and a low transmittivity or poor .gamma. characteristic.